1. Field of the Invention
The present invention relates to a longitudinally-coupled resonator surface acoustic wave (SAW) filter and to a communication apparatus including the same.
2. Description of the Related Art
In recent years, communication apparatuses such as mobile phones often have a transmission frequency band and a reception frequency band, which are near each other, due to increased users and diversification of services. Some communication apparatuses require attenuation that is larger than a predetermined level quite near the passband thereof for preventing interference of other communication apparatuses. Accordingly, SAW filters widely used as a band-pass filter for an RF stage of a mobile phone require larger attenuation than the predetermined level quite near the passband.
For the SAW filters, a function of balance/unbalance signal conversion, which is a so-called balun function, becomes increasingly required in order to reduce the number of parts used. For example, a longitudinally-coupled resonator SAW filter having the function of balance/unbalance signal conversion is disclosed in Japanese Unexamined Patent Application Publication No. 5-267990.
However, when using the known longitudinally-coupled resonator SAW filter described above, a deterioration of the shoulder characteristic, which is called a transversal response (a deterioration of the steepness of frequencies), is seen at the higher frequency-side of the passband. Subsequently, the known longitudinally-coupled resonator SAW filter cannot achieve sufficient attenuation at the higher frequency-side of the passband, which is required for a communication apparatus in the personal communication system (PCS).
For solving such problems, a ladder-type SAW filter is disclosed in Japanese Unexamined Patent Application Publication No. 10-126212, for example. This filter has large attenuation at the higher frequency-side of the passband. However, this ladder-type SAW filter cannot have the function of balance/unbalance signal conversion. That is to say, it is difficult for the known SAW filter to achieve sufficient attenuation at the higher frequency-side of the passband and the function of balance/unbalance signal conversion.
In order to overcome the problems described above, preferred embodiments of the present invention provide a longitudinally-coupled resonator SAW filter which substantially reduces the transversal response compared to the known SAW filter, and which has the function of balance/unbalance signal conversion.
According to a preferred embodiment of the present invention, a longitudinally-coupled resonator surface acoustic wave (SAW) filter includes a piezoelectric substrate and at least two interdigital transducers (IDTs) provided on the piezoelectric substrate along the direction in which a surface acoustic wave propagates. At least one of the IDTs has an electrode finger wherein the metallization ratio of the electrode finger is different from that of other electrode fingers of the IDT.
Preferably, in the longitudinally-coupled resonator SAW filter, at least one of the adjacent IDTs has an electrode finger at a region extending from the adjacent portion of the IDT to an about one-fourth portion of the IDT. The metallization ratio of the electrode finger is preferably different from that of other electrode fingers of the IDT.
Preferably, in the longitudinally-coupled resonator SAW filter, at least one of the IDTs has electrode fingers wherein the metallization ratio of the electrode fingers is continuously varied in the direction along which a surface acoustic wave propagates.
As a result of this unique construction, the longitudinally-coupled resonator SAW filter has reduced transversal response and a large attenuation at the higher frequency-side of the passband. Further, by forming the portion wherein the metallization of the electrode fingers is continuously varied in the direction along which a SAW propagates in the region from the adjacent portion of the IDT to the about one-fourth portion of the IDT, the insertion loss in the passband is not increased. Further, by continuously varying the metallization ratio of the electrode fingers in the surface acoustic propagation direction, the insertion loss within the passband is not increased and a better effect of the present invention can be obtained.
According to another preferred embodiment of the present invention, a communication apparatus includes the longitudinally-coupled resonator SAW filter according to preferred embodiments described above mounted therein. By using the longitudinally-coupled resonator SAW filter of the other preferred embodiments of the present invention, the communication apparatus achieves excellent communication quality and high reliability.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.